1. Field of the Invention
The present invention relates to a high-performance gas chromatograph/mass spectrometer used for quantitative analysis and simultaneous qualitative analysis of trace organic compounds and for structural analysis of sample ions.
2. Description of Related Art
Gas chromatograph/mass spectrometer (GC-MS) instruments fitted with electron impact (EI) ion sources chiefly include magnetic-sector and quadrupole mass spectrometers.
In a magnetic-sector mass spectrometer, a high ion acceleration voltage of the order of kilovolts is usually applied to ions produced in an EI ion source to make them travel toward the mass analyzer region. The ions pass through a free space or a lens system, such as Q lenses, and then are deflected in the mass analyzer region based on the following relation:m/z=K·B2/Vawhere m/z is the mass-to-charge ratio of the ions, B is the magnetic flux density of the magnetic field, Va is the ion acceleration voltage, and K is a constant.
Accordingly, if the magnetic flux density B is kept at a certain value, and if only ions having a certain mass-to-charge ratio are allowed to reach the detector, then a quantitative analysis of the ions can be performed. Furthermore, if the magnetic flux density B is swept, a mass spectrum can be obtained. Hence, simultaneous qualitative analysis can be performed.
On the other hand, in a quadrupole mass spectrometer, ions produced in an EI ion source are made to travel under a low acceleration voltage of about tens of volts. The mass analyzer region of the quadrupole mass spectrometer literally consists of four parallel metal rods which may have hyperbolic or cylindrical surfaces. A superimposition of an RF AC voltage and a DC voltage is applied to each rod to perform mass separation. Based on the following relation, only ions having a certain mass-to-charge ratio pass through the mass analyzer region:m/z=K·Vf/f2 where m/z is the mass-to-charge ratio of the ions, Vf is the RF amplitude voltage, f is the RF frequency, and K is a constant.
In each of the magnetic-sector and quadrupole mass spectrometers, the EI ion source consists of an ionization chamber, source magnets, a filament, and an Einzel lens system made up of plural electrodes. Each of the Einzel lens electrodes is made of a unitary structure provided with a circular opening about the axis of the ion trajectory. See Japanese Patent Laid-Open No. S62-168328.
In recent years, a gas chromatograph/time-of-flight mass spectrometer fitted with an EI ion source has been developed. This instrument has an analyzer region made of an orthogonal acceleration time-of-flight (oa-TOF) mass spectrometer. A time-of-flight mass spectrometer performs mass separation by making use of the fact that the flight speed differs according to the ion mass when a given acceleration voltage is applied to ions. A mass spectrum is recorded according to differences in arrival time at the ion detector.
As this ion detector, a microchannel plate (MCP) detector having high time resolution is often used. In the case of an oa-TOFMS instrument, ions introduced into the ion acceleration region all reach the ion detector regardless of their mass-to-charge ratio.
Therefore, if a carrier gas, such as helium gas, that flows in at a high flow rate of 1 to 2 ml per min. from a gas chromatograph is ionized in large quantity by electron impact, the amount of the resulting ion current reaches 100 to 1,000 times the amount of ion current of the sample ions.
When this large amount of carrier gas ions reaches the MCP, a dead time of tens of microseconds to several milliseconds is produced due to saturation of the MCP. As a result, mass spectral data is lost. In addition, other problems, such as shortening of the life of the MCP, take place.
In conventional magnetic-sector and quadrupole mass spectrometers, only ions having a certain mass-to-charge ratio are allowed to reach the detector, and mass separation is effected. Because of this mechanism, it has been possible to prevent the ions of the carrier gas of the gas chromatograph from reaching the detector by appropriately setting the analytical conditions. Therefore, it can be understood that these problems are unique to TOFMS instruments. Furthermore, where ions are treated under low acceleration conditions, the effects of charging due to adhesion of contaminants to the electrodes cannot be neglected.
For example, FIG. 1 shows the configuration of an orthogonal acceleration time-of-flight mass spectrometer. This instrument has an external ion source 1, a differentially pumped (evacuated) chamber 10 formed by first and second partition walls and by a vacuum pump (not shown), an intermediate chamber 11, and a measuring chamber 13. A first orifice 2 is formed in the first partition wall of the differentially pumped chamber 10. A ring lens 3 is placed in the differentially pumped chamber 10. A second orifice 4 is formed in the second partition wall of the differentially pumped chamber 10. Ion guides 5 are placed in the intermediate chamber 11. Ion optics including a set of lenses 6 made up of focusing lenses and a deflector, a launcher 7 consisting of an ion repeller plate and accelerating lenses (grids), an ion reflector 8 for reflecting ions, and an ion detector 9 are fitted in the measuring chamber 13.
In this configuration, ions produced from a sample in the external ion source 1 are first introduced into the differentially pumped chamber 10 through the first orifice 2. The ions that tend to diffuse themselves in the differentially pumped chamber 10 are focused by the ring lens 3 located inside the chamber 10 and admitted into the intermediate chamber 11 through the second orifice 4. The ions are then decreased in kinetic energy in the intermediate chamber 11. The diameter of the ion beam is reduced by the RF electric field produced by the ion guides 5 and guided into the high-vacuum measuring chamber 13. A third orifice 12 is formed in the partition wall that partitions the intermediate chamber 11 and measuring chamber 13 from each other. The ions guided in from the ion guides 5 are shaped into a round ion beam having a certain diameter by the third orifice 12 and introduced into the measuring chamber 13.
The set of lenses 6 consisting of the focusing lenses and deflector is installed at the entrance of the measuring chamber 13. The ion beam entering the measuring chamber 13 is corrected in terms of diffusion and deflection by the lenses 6 and then introduced into the launcher 7. As shown in FIG. 2, an ion reservoir and accelerating lenses arrayed perpendicularly to the axis of the ion reservoir are mounted in the launcher 7. In the ion reservoir, an ion repeller electrode 14 and grid 15 are placed opposite to each other.
The ion beam 18 is at first in a very low energy state of 20 to 50 eV and is introduced parallel toward the ion reservoir 17 surrounded by the ion repeller electrode 14, grids 15, and accelerating lenses 16 as shown in FIG. 2. The ion beam 18 having a given length and moving parallel through the ion reservoir 17 is pulsed and accelerated in a direction (Z-axis direction) perpendicular to the direction of entrance (X-axis direction) of the ion beam 18 by applying a pulsed accelerating voltage on the order of kV having the same polarity as the ions to the repeller plate 14 as shown in FIG. 2. As a result, an ion pulse 19 that starts to travel toward a reflector 8 located opposite to the ion reservoir 17 is formed.
The ions accelerated in the vertical direction have a velocity that is the sum of the X-axis direction velocity assumed when they are introduced into the measuring chamber 13 and the Z-axis direction velocity that is given perpendicularly to the X-axis direction by the ion repeller electrode, grids, and accelerating lenses. Consequently, the ions travel in a direction slightly deviating from the Z-axis direction and are reflected into the ion detector 9 by the reflector 8.
In the oa-TOFMS instrument, ions must be introduced into the ion reservoir 17 with a quite low accelerating energy. Because of this principle, it is advantageous to set the ion acceleration voltage at the ion source as low as possible. If such ion introduction at low velocity is carried out using a large amount of carrier gas ions, the ions come into contact with the ion repeller electrode 14 and grids 15 because the ions have a spatial spread. This promotes charging of these electrodes.
In the case of an oa-TOFMS instrument, mass separation is performed based on variations in arrival time at the ion detector. Therefore, the instrumental sensitivity and resolution depend on the initial position of the ions in the ion reservoir and on the uniformity of the initial kinetic energy. Accordingly, if there is nonuniform charging in the ion reservoir, the ions introduced with low acceleration have nonuniform initial position in the ion reservoir and nonuniform initial kinetic energy. Consequently, the instrumental sensitivity and resolution are deteriorated severely. Furthermore, they will age with time.